The inkjet printing of substrates intended for direct contact with food or skin requires careful consideration of the toxicity and the potential for unintended side effects from human exposure to the chemicals contained in the inkjet inks. The most toxic chemicals in aqueous inkjet ink compositions tend to be the industrial biocides used to prevent microbial growth of bacteria, yeasts, and molds to extend composition shelf-life. Isothiazolinones and formaldehyde released from precursors or donors, for example, are generally safe and have high antimicrobial efficacy within their prescribed usage levels. Nonetheless, such compounds have been recently found objectionable in certain products intended for skin contact because sensitive users can experience skin irritation and dermatitis following exposure to even trace levels. Regulatory bodies and advocacy groups are becoming more influential in what chemicals are used in such consumer products. In addition, the population of consumers is becoming more aware of the use of common preservatives in various products and are developing a preference for products that do not contain them.
A need remains to provide alternative preservatives (or biostats and biocides) with a record of safety and acceptance in skin contact applications that are effective at suppressing microbial contaminant growth in aqueous inkjet ink compositions without destabilizing pigment or dye-based colorants or compromising the high-quality printing process in some other fashion.
Piezoelectric drop-on-demand (DOD) inkjet inks are disclosed in U.S. Patent Application Publication 2017/0073535A1 (Kusukame et al.) using selections of conventional and unconventional components.
The use of 2-phenoxyethanol (identified as ethylene glycol phenyl ether) in continuous inkjet printing (CIJ) processes using aqueous dye-based and pigment-based inks is disclosed in U.S. Pat. No. 6,071,334 (Wider et al.).
Ethylene glycol phenyl ether is described as a substantially water insoluble oil at about 3 weight % in thermal DOD black inkjet inks comprised of self-dispersed carbon black pigments, as disclosed in U.S. Pat. No. 5,749,952 (Tsang et al.)
Pigmented thermal DOD inkjet ink compositions using self-dispersed black pigment and acrylate polymer binder experience reduced “deceleration” (loss of drop velocity during periods of extended nozzle firing) if a deceleration-alleviating component selected from a group of materials including ethylene glycol phenyl ether is incorporated in the ink as described in U.S. Pat. No. 6,726,757B2 (Sarkisian et al.).
The use of 2-phenylethanol (identified as 0-phenylethyl alcohol) in a fragrant aqueous inkjet ink composition intended to impart rose scent is disclosed in U.S. Pat. No. 6,123,757 (Yang et al.).
Despite these various innovations, there is an unmet need for aqueous inkjet ink compositions and particle-free fluids that are substantially free of common preservatives or biocides, while preserving desired continuous inkjet printing (CIJ) features and high-quality images.